In recent years, a number of medical devices have been designed which are adapted for compression into a small size to facilitate introduction into a vascular passageway and which are subsequently expandable so as to make contact with the walls of the passageway. These devices include, among others, blood clot filters which expand and are held in position by engagement with the inner wall of a blood vessel, such as a vein and in particular the vena cava. These vena cava filters have been designed to remain in place permanently. Such filters include structure to secure the filter within the vena cava, such as elongate diverging anchor members with hooked ends that penetrate the vessel wall and positively prevent longitudinal migration in either direction within the vessel. The hooks on filters of this type are rigid and will not bend, and within two to six weeks after a filter of this type has been implanted, the endothelium layer grows over the diverging anchor members and positively locks the hooks in place. Thereafter, any attempt to remove the filter results in a risk of injury to the vena cava, including potential rupture.
A number of conditions and medical procedures subject the patient to a short term risk of pulmonary embolism which can be alleviated by a filter implant. In such cases, patients are often averse to receiving a permanent implant because the risk of pulmonary embolism may disappear after a period of several weeks or months. However, not all existing filters are conducive to removal after they have been implanted for more than two weeks. Moreover, most existing filters have different consequences on the vessel wall during removal subsequent to the two-week time period. For example, a known filter is believed to cause the vessel wall diameter to collapse substantially upon removal of such filter due to the attachment of the hooks to the vessel wall.
In an attempt to provide a removable filter, two filter baskets have been formed along a central shaft that are conical in configuration, with each basket being formed by spaced struts radiating outwardly from a central hub. The central hubs are held apart by a compression unit, and the locator members of the two baskets overlap so that the baskets face one another. Filters of this type require the use of two removal devices inserted at each end of the filter to draw the baskets apart and fracture the compression unit. The end sections of the locator members are formed to lie in substantially parallel relationship to the vessel wall and the tips are inclined inwardly to preclude vessel wall penetration. If a device of this type is withdrawn before the endothelium layer grows over the locator members, vessel wall damage is minimized. But after the endothelium layer grows over the locators, the combined inward and longitudinal movement of the filter baskets as they are drawn apart can tear this layer.
Filters designed to be removable have been disclosed, such as in U.S. Pat. Nos. 6,007,558 and 6,258,026. These disclosed filters feature anchor members 30 which are fabricated so the hook portion has a cross section smaller than that of the rest of the anchor member, as illustrated in FIGS. 22 and 23A. So configured, when an extraction tool pulls on the filter hub 10, the force is transferred through the anchor member to the hook which, implanted in the vessel wall, preferentially bends as illustrated in FIG. 23B. So bent, the hooks may exit without rupturing the vessel wall. Nevertheless, if the filter has been in the blood vessel for several weeks, the endothelial overgrowth may be substantial enough to be vulnerable to lateral movement of the anchor and hook. As illustrated in FIG. 24, with the hook 40 in a bent configuration, tension 8 along the anchor 30 will result in a lateral force 9 which may tend to rip the hook 40 and lower portion of the anchor through the endothelial overgrowth 7. If the endothelial overgrowth 7 does not easily tear or split, the overgrowth may complicate removal of the filter or the blood vessel may be damaged.